Composite materials are increasingly used in the manufacturing process for a wide variety of products that require a high strength-to-weight ratio. A composite is a material made up of two or more components that confer different types of strength or resilience. For example, carbon fibers having a high tensile strength are embedded in an epoxy resin. The combination produces a material that may be formed to a wide variety of shapes and which, pound for pound, is many times stronger than steel.
In a typical composite manufacturing process, many long strands of carbon or other fibers are pulled from spools and aligned so that they are parallel. The parallel band of fibers is then applied and pressed to a heated surface and held in place with an epoxy resin. Because the resulting composite material is strongest in the direction that the fibers run, several layers of fibers are applied in the composite manufacturing process. In this layering, or laminating, process layers of composite fibers are applied so that the fibers in successive layers run transversely of the fibers of preceding layers to produce a material that is strong in every direction. Once the composite has been built up, it is heat cured in a kiln or autoclave, producing a rigid structure.
Although the composite laminate process is extremely effective at producing strong, lightweight materials, problems can occur in the manufacturing process that affect the strength of the resulting material. First, gaps can occur between the carbon fibers that are applied to the surface. Such gaps are especially likely to occur when producing a composite device having numerous bends, folds, or curves. The presence of gaps of significant size can weaken the resulting composite material in the area of the gaps. This weakness is compounded when there are multiple gaps in the same general vicinity. Similarly, fibers that are intended to be adjacent and parallel may overlap one another. The overlapping fibers, or "laps", also create an area of decreased strength. The presence of a lap may even be worse than the presence of a gap. Because the fibers expand during the heat curing process, gaps below a certain size will become filled as the fibers expand and shift toward one another. A lap, however, is likely to worsen during the curing process. An overlapping fiber will typically remain in place but will become stretched as the fibers expand during the curing process. As a result, a lap creates a weak area in the composite material in the vicinity of the lap.
To avoid these weakened areas in the finished material, each layer of fibers must be inspected for laps and gaps. Typically, inspectors visually survey the surface of a composite material looking for laps and gaps. This is an especially tedious process that is prone to error. Because the fibers and gaps are small, a magnifying glass must be used to search for laps and gaps. In addition, because carbon fibers are black, it is particularly difficult to visually find a lap or gap in the top layer of fibers when the layer of fibers below it is also black. Moreover, it is difficult to precisely measure the size of a gap between fibers, making it difficult to determine whether an identified gap is sufficiently large to pose a problem. These problems are compounded when the composite material being manufactured is large. An inspector searching the surface of a large composite material is prone to fatigue and eye strain and is likely to miss significant laps and gaps.